
SCIENCE84
7.5 Reproductive Phase of
Life in Humans
Adolescents become capable of
reproduction when their testes and
ovaries begin to produce gametes. The
capacity for maturation and production
of gametes lasts for a much longer time
in males than in females.
In females, the reproductive phase of
life begins at puberty (10 to 12 years of
age) and generally lasts till the age of
approximately 45 to 50 years. The ova
begin to mature with the onset of
puberty. One ovum matures and is
released by one of the ovaries once in
about 28 to 30 days. During this
period, the wall of the uterus becomes
thick so as to receive the egg, in case
it is fertilised and begins to develop.
This results in pregnancy. If
fertilisation does not occur, the
released egg, and the thickened lining
of the uterus along with its blood
vessels are shed off. This causes
bleeding in women which is called
menstruation. Menstruation occurs
once in about 28 to 30 days. The first
menstrual flow begins at puberty and
is termed menarche. At 45 to 50 years
of age, the menstrual cycle stops.
Stoppage of menstruation is termed
menopause. Initially, menstrual cycle
may be irregular. It take some time to
become regular.
Menstrual cycle is controlled by
hormones. The cycle includes the
maturation of the egg, its release,
thickening of uterine wall and its
breakdown if pregnancy does not
occur. In case the egg is fertilised it
begins to divide and then gets
embedded in the uterus for further
development as you have learnt in
Chapter 6 (Fig. 6.8).
7.6 How is the Sex of the
Baby Determined?
I heard my mother and my
aunt talking about my cousin
who is going to have a baby.
They were discussing whether
she would give birth to a boy
or a girl. I wonder what makes
the fertilised egg develop
either into a boy or a girl!
Paheli says that the
reproductive life of a
woman lasts from
menarche to menopause.
Is she right?
Boy or Girl?
Inside the fertilised egg or zygote is the
instruction for determining the sex of
the baby. This instruction is present in
the thread-like structures, called
chromosomes in the fertilised egg.
Chromosomes are present inside the
nucleus of every cell. All human beings
have 23 pairs of chromosomes in the
nuclei of their cells. Two chromosomes
out of these are the sex chromosomes,
named X and Y. A female has two X
chromosomes, while a male has one X
and one Y chromosome. The gametes
(egg and sperm) have only one set of
chromosomes. The unfertilised egg
always has one X chromosome. But